Is my coworker ing in all of the drinks at Starbucks?

There's a new guy who was just hired this week and I suspect that he is ing in the milk at Starbucks. He constantly going to the back room to refill the milk jugs and always comes back sweaty. Once I even caught his fly unzipped. I've had customers complain about the taste of their coffee and when I confronted him on this he told me "must have been spoiled milk"...What should I do?